[Java] Remove Static imports for Utf8 Usage

[vibhatha]

Describe the enhancement requested

With the errorprone warning fixes, a decision was made to minimize the static imports. And a few more classes which does the UTF8 import in static mode was found in FlightSqlExample, JsonFileReader ArrowFlightJdbcVarCharVectorAccessor.
It would be good to fix these.

Component(s)

Java

[vibhatha]

I will continue to work on these improvements following up reviews in #39529

[vibhatha]

I will continue to work on these improvements following up reviews in #39529

I have missed this unfortunately. Creating a smaller PR to fix this.

[lidavidm]

Issue resolved by pull request 40683
#40683